Improve array/hash building syntax.

[armstnp]

Right now, nested array and hash validations lead to a 'paren cascade' - which, while it may be suitable for a Lisp, looks quite uncomfortable in Ruby.

A more robust instance_eval-based builder may be sufficient to make construction of these validations more compact and more declarative.

This syntax would probably depend on #7.

Possible example of syntax:

hash {
  before { rule(error: :invalid_key) { !_1.key? :bad_key } }

  mandatory(:field_a) { typed(T::Decimal) > gteq(0) & lteq(100) }
  optional(:field_b) { typed(T::String) }

  after { rule(error: :mismatch) { _1[:field_b].nil? || _1[:field_a].to_s == _1[:field_b] } }
}